CYANIDE REMOVAL FROM WASTWATER OF GOLD LEACHING PROCESS

Cyanide is frequently used in gold mines for an extraction process called “Leaching” or “Gold cyanidation” that consists on the separation of the soluble component (gold) from the solid mass (mineral) by using a solvent (Cyanide). It is a simple and economic way for mineral processing to recover gold, copper, zinc and silver; but it has created great controversy in the last decade due to its high impact on the environment.

For the leaching process (a procedure that must take place outdoors), the miners drop a cyanide-based solution on huge pieces of mineral that contains gold, and after the reaction has taken place, the leaching solution is collected into tanks or containers; this solution is applied a certain amount of times on the mineral piece and in this way the gold separation is obtained.

PROBLEMS

The biggest disadvantage of this procedure is the fact that cyanide is extremely toxic and lethal for living organisms. For example, a person can be warned not to drink wastewater from this process but nothing prevents birds or mammals from drinking this water when it is stored in the collection tanks or after it has been discharged on rivers and seas. Furthermore, this collection tanks may have leaks or overflows that can have a huge impact on the sea life creatures and natural flora of rivers and lakes.

On the other hand, when the cyanide has destroyed the heavy metals, it can form other metal complex or chemical solutions that can be as lethal as the cyanide itself. 

THE AQUATIC FAUNA

All the fish and aquatic invertebrates  are particularly sensitive to the cyanide exposition since it inhibits the oxygen absorption on the cells and in this way it makes the animals suffocate. The aquatic organisms can resist up to a certain concentration of cyanide (usually a few micrograms per liter); the birds and terrestrial mammals resist up to a few milligrams per liter.

Concentrations of about 5.0 and 7.2 micrograms per liter on the waterbody reduces the mobility and inhibits the reproduction in most species of fish. Other negative effects are the delayed mortality, sickness, predisposition to predation, interrupted breathing, osmoregulatory dispersion and disturbed growth patterns.  The degree of sensitivity of the aquatic organisms depends on the species and it is highly influenced by the water pH, temperature, oxygen level and the developing state of the animal.

THE AQUATIC FLORA

The algae and macrophytes can tolerate higher levels of cyanide than fish and invertebrates; they usually demonstrate the negative effects when concentrations reach 160 micrograms per liter or more. The aquatic plants are normally not affected by levels of cyanide that are lethal to fish and aquatic invertebrates.

ANAEROBIC ENVIRONMETS

Under anaerobic conditions, the microbial activity can degrade the cyanide to ammonia, that is further ahead oxidized into nitrate. Such processes are efficient when cyanide concentration reaches 200 micrograms per liter.

There are some technologies particularly developed for the cyanide removal from water, such as:

• Oxidation with alkaline chlorination
• Oxidation with ozone and UV rays
• Oxidation with peroxide.